What is a reason to implement PIM-DM as opposed to PIM-SM?

What is a reason to implement PIM-DM as opposed to PIM-SM?
A . to control exactly which multicast groups are routed through the network
B . to permit a higher density of RP routers in the network core
C . to conserve bandwidth over WAN links
D . to use on high-bandwidth routed connections

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Answer: D